Magnetic disc drives have read/write heads which are used for both writing data to a magnetic disc and reading data from the magnetic disc. During a write operation, a write signal is provided to a selected read/write head from a write control circuit. The write signal represents data to be encoded into the magnetic disc. More particularly, the read/write head receives encoded digital data from a "channel" chip. The transitions of the signal received from the channel chip cause the write current flowing within the write/read head to reverse direction which in turn, induces a flux reversal in the magnetized material of the medium.
During a read operation, the read/write head senses flux reversals from the magnetic disc. The flux reversals are encoded into the magnetic disc during the write operation. Based on the flux reversals, the read/write head provides a read signal to a read channel. The read circuit amplifies the read signal, and the channel circuit recovers the data. The read circuit then provides the data to a magnetic disc controller for further processing.
Each magnetic disc in a disk drive has a corresponding "head" adjacent to the top and bottom surface of the disc. Thus, there are two N heads per stack where N equals the number of disks in a drive. Normally, only one head is active at a given time in order to control these channels.
Each channel additionally includes a current path for current to flow to the MR head. In this current path is an input transistor to control the flow of the current in the current path, the input transistor is controlled by connecting the base of the transistor to a switch which can be selectively activated. Since each of the channels have one of these input transistors, when one channel is desired to be inactivated, the switch disconnects the base of the input transistor of that channel while another channel is activated by connecting the base of input transistor of this other channel to a voltage, allowing the current to flow the activated current path. The bases of these input transistors are usually connected to a common bias line which is coupled to a capacitor to stabilize and noise filter the voltage in the line. However, this capacitor can introduce problems. Typically, the resistance of the individual magnetic head varies from channel to channel. However, the bias current in the current path of the channel is generally constant, resulting in differing voltages across the magnetic head. If there is a large resistance with a constant current a large voltage results. A small resistance with the same constant current results in a small voltage. These voltages are on the capacitor. As the channels are switched, the voltage on the capacitor can adversely affect the current through the magnetic lead switching from a channel with a high resistance to a channel with a low resistance causing excess current to flow in the channel which has been activated.
FIGS. 1a-1e illustrates the problem resulting from the switching between channels of differing resistance. Presuming that switching has occurred between the second and third channels at t.sub.1, the current I.sub.B2 through the current path of the second channel is reduced to zero while as illustrated in FIG. 1d, the current I.sub.B3 at t.sub.1 is the current flowing through the third channel based on the resistance R.sub.MR3 of the third channel. Correspondingly, as illustrated in FIG. 1e, the voltage across V.sub.c increases at t=1 to a V.sub.c2 which is larger than V.sub.c1 as a result of the higher resistance R.sub.3. However, a problem develops as illustrated in FIG. 1c at t.sub.2 when the MR system switches from a channel having high resistance such as the third channel to a channel having low resistance such as the second channel. Because of the high voltage V.sub.c2 the current is determined by the instant amendment the claims have been amended to obviate the rejection relations of the voltage to the head resistance. Thus, a high voltage results in, as illustrated in FIG. 1c, a high current beginning at time t.sub.2 and ending at a time subsequent to t.sub.2 when the voltage reaches the C.sub.1. This high current for the interval time after t.sub.2 results in damage to the head associated with the second channel. Thus, there is a need to eliminate the high transient current resulting from switching between channels, particularly when there is a switch from a channel having a relatively high impedance to a channel having a relatively low impedance.